This invention relates to a method and apparatus for controlling the speed of a motor. In particular, the invention relates to using a command signal to control motor speed through an exponential relationship between the signal and the desired speed.
There are a number of instances in which it is desirable to maintain a predetermined speed of a motor. This motor may be used to drive, for example, a vehicle, a conveyor belt, or a robotic component of a mechanized factory system. Motors are presently available which are capable of maintaining constant speed, however, this characteristic is not ordinarily compatible with the ability to select over a wide range of constant speeds. The present invention provides a means whereby a preselected motor speed can be maintained within narrow limits and wherein this speed may be chosen over a wide range of possible RPMs.
A particularly demanding application is that wherein the motor is used to drive a pump, which pump supplies a fluid for, for example, a chemical reaction or for a fermenter. Many modern industrial processes in the chemical or bio-engineering fields involve mixing of fluid constituents to form solutions of one type or another. The mixing is generally accomplished by pumping the fluid constituents into production vessels under carefully regulated conditions. The fluid pumps are driven by pump motors controlled to provide the fluid flow rates optimally associated with the processes. Conventional pump motor control techniques often rely on signals which are generated in response to process sensor outputs and which exhibit control voltages proportional to the optimal or desired fluid flow rates. The control voltages are converted into control frequencies suitable for use in driving the pump motors at the speeds required to establish the desired fluid flow rates. The ability of such prior art pump motor control techniques to provide accurate control over the amount of a given fluid admitted to a production vessel depends to a great extent on the nature of the control voltage-to-frequency conversion function. Many prior art pump motor controllers employ linear conversion functions, whereby the sensitivity of the pump motor control circuitry to indicated changes in the desired flow rate remains constant over the entire range of flow rates available to the pump.
Many industrial processes can be affected greatly by even small changes in the quantities of constituent fluids admitted to the production vessels. In order to maintain proper pH levels in a fermentation culture, for example, a base or acid must be supplied to the culture in precisely measured amounts at rates which vary continuously in accordance with the measured pH level of the culture. The linear conversion function for process control signals in existing pumps does not yield the pump motor speed resolution necessary to achieve fine control over base or acid supply at the low flow rates typically encountered during fermentation pH level adjustments. Under these circumstances, it would be particularly beneficial to provide a pump motor controller capable of performing control voltage-to-frequency conversions, in a manner which permits high resolution of pump motor speed settings at low fluid flow rates. The present invention offers just such an advantage.